Example of random-access memory: Synchronous, primarily used as main memory in, and.Random-access memory ( RAM ) is a form of that can be read and changed in any order, typically used to store working. A memory device allows items to be or written in almost the same amount of time irrespective of the physical location of data inside the memory. In contrast, with other direct-access data storage media such as, and the older and, the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.RAM contains and circuitry, to connect the data lines to the addressed storage for reading or writing the entry. Usually more than one bit of storage is accessed by the same address, and RAM devices often have multiple data lines and are said to be '8-bit' or '16-bit', etc. Devices.In today's technology, random-access memory takes the form of (IC) chips with (metal-oxide-semiconductor). RAM is normally associated with types of memory (such as ), where stored information is lost if power is removed, although non-volatile RAM has also been developed. Other types of exist that allow random access for read operations, but either do not allow write operations or have other kinds of limitations on them.

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These include most types of and a type of called.The two main types of volatile random-access are (SRAM) and (DRAM). Commercial uses of semiconductor RAM date back to 1965, when IBM introduced the SP95 SRAM chip for their computer, and used DRAM memory cells for its Toscal BC-1411, both based on.

Commercial MOS memory, based on, was developed in the late 1960s, and has since been the basis for all commercial semiconductor memory. The first commercial DRAM IC chip, the, was introduced in October 1970. (SDRAM) later debuted with the KM48SL2000 chip in 1992. 1 (MB) chip, one of the last models developed by in 1989.Early computers used, or for main memory functions. Ultrasonic delay lines could only reproduce data in the order it was written. Could be expanded at relatively low cost but efficient retrieval of memory items required knowledge of the physical layout of the drum to optimize speed. Latches built out of, and later, out of discrete, were used for smaller and faster memories such as registers.

Such registers were relatively large and too costly to use for large amounts of data; generally only a few dozen or few hundred bits of such memory could be provided.The first practical form of random-access memory was the starting in 1947. It stored data as electrically charged spots on the face of a.

Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access. The capacity of the Williams tube was a few hundred to around a thousand bits, but it was much smaller, faster, and more power-efficient than using individual vacuum tube latches. Developed at the in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the computer, which first successfully ran a program on 21 June 1948. In fact, rather than the Williams tube memory being designed for the Baby, the Baby was a to demonstrate the reliability of the memory.was invented in 1947 and developed up until the mid-1970s. It became a widespread form of random-access memory, relying on an array of magnetized rings.

By changing the sense of each ring's magnetization, data could be stored with one bit stored per ring. Since every ring had a combination of address wires to select and read or write it, access to any memory location in any sequence was possible. Magnetic core memory was the standard form of system until displaced by in (ICs) during the early 1970s.Prior to the development of integrated (ROM) circuits, permanent (or read-only) random-access memory was often constructed using driven by, or specially wound planes. began in the 1960s with bipolar memory, which used. While it improved performance, it could not compete with the lower price of magnetic core memory.

MOS RAMThe invention of the (metal-oxide-semiconductor field-effect transistor), also known as the MOS transistor, by and at in 1959, led to the development of (MOS) memory by John Schmidt at in 1964. In addition to higher performance, MOS was cheaper and consumed less power than magnetic core memory. The development of (MOS IC) technology by at Fairchild in 1968 enabled the production of MOS.

MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s.An integrated bipolar (SRAM) was invented by Robert H. Norman at in 1963. It was followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964. SRAM became an alternative to magnetic-core memory, but required six MOS transistors for each of data.

Commercial use of SRAM began in 1965, when introduced the SP95 memory chip for the.(DRAM) allowed replacement of a 4 or 6-transistor latch circuit by a single transistor for each memory bit, greatly increasing memory density at the cost of volatility. Data was stored in the tiny capacitance of each transistor, and had to be periodically refreshed every few milliseconds before the charge could leak away. 's Toscal BC-1411, which was introduced in 1965, used a form of capacitive bipolar DRAM, storing 180-bit data on discrete, consisting of bipolar transistors and capacitors. While it offered improved performance over magnetic-core memory, bipolar DRAM could not compete with the lower price of the then dominant magnetic-core memory.MOS technology is the basis for modern DRAM. At the was working on MOS memory. While examining the characteristics of MOS technology, he found it was capable of building, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor.

This led to his development of a single-transistor DRAM memory cell. In 1967, Dennard filed a patent under IBM for a single-transistor DRAM memory cell, based on MOS technology. The first commercial DRAM IC chip was the, which was on an MOS process with a capacity of 1, and was released in 1970.(SDRAM) was developed. The first commercial SDRAM chip was the Samsung KM48SL2000, which had a capacity of 16. It was introduced by in 1992, and mass-produced in 1993. The first commercial ( SDRAM) memory chip was Samsung's 64 DDR SDRAM chip, released in June 1998.

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(graphics DDR) is a form of DDR (synchronous graphics RAM), which was first released by Samsung as a 16 Mb memory chip in 1998. TypesThe two widely used forms of modern RAM are (SRAM) and (DRAM). In SRAM, a is stored using the state of a six-, typically using six (metal-oxide-semiconductor field-effect transistors). This form of RAM is more expensive to produce, but is generally faster and requires less dynamic power than DRAM.

In modern computers, SRAM is often used as. DRAM stores a bit of data using a transistor and pair (typically a MOSFET and, respectively), which together comprise a DRAM cell. The capacitor holds a high or low charge (1 or 0, respectively), and the transistor acts as a switch that lets the control circuitry on the chip read the capacitor's state of charge or change it.

As this form of memory is less expensive to produce than static RAM, it is the predominant form of computer memory used in modern computers.Both static and dynamic RAM are considered volatile, as their state is lost or reset when power is removed from the system. By contrast, (ROM) stores data by permanently enabling or disabling selected transistors, such that the memory cannot be altered. Writeable variants of ROM (such as and ) share properties of both ROM and RAM, enabling data to without power and to be updated without requiring special equipment. These persistent forms of semiconductor ROM include flash drives, memory cards for cameras and portable devices, and.(which can be either SRAM or DRAM) includes special circuitry to detect and/or correct random faults (memory errors) in the stored data, using or.In general, the term RAM refers solely to solid-state memory devices (either DRAM or SRAM), and more specifically the main memory in most computers. In optical storage, the term is somewhat of a misnomer since, unlike or it does not need to be erased before reuse. Nevertheless, a DVD-RAM behaves much like a hard disc drive if somewhat slower.Memory cell.

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Main article:The memory cell is the fundamental building block of. The memory cell is an that stores one of binary information and it must be set to store a logic 1 (high voltage level) and reset to store a logic 0 (low voltage level). Its value is maintained/stored until it is changed by the set/reset process.

The value in the memory cell can be accessed by reading it.In SRAM, the memory cell is a type of circuit, usually implemented using. This means that SRAM requires very low power when not being accessed, but it is expensive and has low storage density.A second type, DRAM, is based around a capacitor.

Charging and discharging this capacitor can store a '1' or a '0' in the cell. However, the charge in this capacitor slowly leaks away, and must be refreshed periodically. Because of this refresh process, DRAM uses more power, but it can achieve greater storage densities and lower unit costs compared to SRAM. DRAM Cell (1 Transistor and one capacitor)AddressingTo be useful, memory cells must be readable and writeable. Within the RAM device, multiplexing and demultiplexing circuitry is used to select memory cells. Typically, a RAM device has a set of address lines A0. An, and for each combination of bits that may be applied to these lines, a set of memory cells are activated.

Due to this addressing, RAM devices virtually always have a memory capacity that is a power of two.Usually several memory cells share the same address. For example, a 4 bit 'wide' RAM chip has 4 memory cells for each address. Often the width of the memory and that of the microprocessor are different, for a 32 bit microprocessor, eight 4 bit RAM chips would be needed.Often more addresses are needed than can be provided by a device. In that case, external multiplexors to the device are used to activate the correct device that is being accessed.Memory hierarchy. Main article:One can read and over-write data in RAM.

Many computer systems have a memory hierarchy consisting of, on-die caches, external, systems and or on a hard drive. This entire pool of memory may be referred to as 'RAM' by many developers, even though the various subsystems can have very different, violating the original concept behind the random access term in RAM.

Even within a hierarchy level such as DRAM, the specific row, column, bank, channel, or organization of the components make the access time variable, although not to the extent that access time to rotating or a tape is variable. The overall goal of using a memory hierarchy is to obtain the highest possible average access performance while minimizing the total cost of the entire memory system (generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom).In many modern personal computers, the RAM comes in an easily upgraded form of modules called or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or when changing needs demand more storage capacity. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the and other on the, as well as in hard-drives, and several other parts of the computer system.Other uses of RAM. Main article:Most modern operating systems employ a method of extending RAM capacity, known as 'virtual memory'. A portion of the computer's is set aside for a paging file or a scratch partition, and the combination of physical RAM and the paging file form the system's total memory. (For example, if a computer has 2 GB of RAM and a 1 GB page file, the operating system has 3 GB total memory available to it.) When the system runs low on physical memory, it can ' portions of RAM to the paging file to make room for new data, as well as to read previously swapped information back into RAM.

Excessive use of this mechanism results in and generally hampers overall system performance, mainly because hard drives are far slower than RAM.RAM disk. Main article:Software can 'partition' a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a. A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source.Shadow RAMSometimes, the contents of a relatively slow ROM chip are copied to read/write memory to allow for shorter access times. The ROM chip is then disabled while the initialized memory locations are switched in on the same block of addresses (often write-protected). This process, sometimes called shadowing, is fairly common in both computers and.As a common example, the in typical personal computers often has an option called “use shadow BIOS” or similar.

When enabled, functions that rely on data from the BIOS's ROM instead use DRAM locations (most can also toggle shadowing of video card ROM or other ROM sections). Depending on the system, this may not result in increased performance, and may cause incompatibilities. For example, some hardware may be inaccessible to the if shadow RAM is used. On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Free memory is reduced by the size of the shadowed ROMs. Recent developmentsSeveral new types of, which preserve data while powered down, are under development. The technologies used include and approaches utilizing.

Amongst the 1st generation, a 128 ( 128 × 2 10 bytes) chip was manufactured with 0.18 µm technology in the summer of 2003. In June 2004, unveiled a 16 (16 × 2 20 bytes) prototype again based on 0.18 µm technology. There are two 2nd generation techniques currently in development: (TAS) which is being developed by, and (STT) on which, and several other companies are working. Built a functioning carbon nanotube memory prototype 10 (10 × 2 30 bytes) array in 2004. Whether some of these technologies can eventually take significant market share from either DRAM, SRAM, or flash-memory technology, however, remains to be seen.Since 2006, ' (based on flash memory) with capacities exceeding 256 gigabytes and performance far exceeding traditional disks have become available. This development has started to blur the definition between traditional random-access memory and 'disks', dramatically reducing the difference in performance.Some kinds of random-access memory, such as 'EcoRAM', are specifically designed for, where is more important than speed.

Memory wallThe 'memory wall' is the growing disparity of speed between CPU and memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries, which is also referred to as bandwidth wall. From 1986 to 2000, speed improved at an annual rate of 55% while memory speed only improved at 10%.

Given these trends, it was expected that memory latency would become an overwhelming in computer performance.CPU speed improvements slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Summarized these causes in a 2005 document.First of all, as chip geometries shrink and clock frequencies rise, the transistor increases, leading to excess power consumption and heat. Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called ), further undercutting any gains that frequency increases might otherwise buy.

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